DNA and RNA helicases represent a diverse class of enzymes that couple the energy of nucleotide hydrolysis to unwinding of nucleic acid double helices. These helicases represent an important class of molecular motors whose activities are critical for RNA processing, translation, DNA synthesis and DNA repair. The 40kDa RuvB DNA helicase is intimately linked to DNA repair. The ATP-hydrolyzing RuvB helicase is targeted to Holiday junctions by the RuvA tetramer. Together, this RuvAB complex not only migrates to branches of the Holiday junctions, but also displaces RecA filament from DNA strands. The entire class of helicases are an ancient class of enzymes; however, no structures currently exist. We seek to solve the structure of RuvB as the first in a class of helicase and use it to understand how small scale movements involved in ATP nydrolysis are linked to large scale motions.